1. Field of the Invention
This invention relates to novel diamino compounds and methods for preparing them.
2. Description of the Related Art
For liquid crystal display elements used in watches and electronic computers, there is mainly adopted a twist nematic (hereinafter referred to TN) mode in which aligning directions of nematic liquid crystal molecules being twisted by 90 degrees between one pair of upper and lower two electrode substrates. Also, there has been developed a super twisted nematic (hereinafter referred to STN) mode in which a twist angle being increased to 180-300 degrees and thus liquid crystal display elements with good display quality can be obtained even for a large image plane.
Furthermore, since matrix displays and color displays etc. have been recently realized, there have been actively developed MIM (metal-insulator-metal) elements and TFT (field-effect mode thin film transistor) elements adopting a number of selective electrodes and an active type twist nematic mode by which their ON-OFF actions can be carried out.
As a problem common to all these modes, a phenomenon called an image sticking is caused, that is, after displaying the same image for a long term, the previous image is left as an after-image at the time of changing to the next image. Particularly, in order to obtain liquid crystal display elements with high quality, it becomes a very important problem to improve the said image sticking.
The cause of the image sticking is considered to be an electric potential difference produced between the upper and lower substrates by a deviation of electric charge and kept stably, and the said deviation is caused by production of electric double layers due to ion components of impurities contained in the liquid crystal at an aligning film surface by a DC component applied On a liquid crystal element. Particularly, in TFT element, since a DC component cannot be eliminated owing to characteristics of the element, the image sticking is more eminent and serious than in TN and STN elements.
Also in a TFT mode, in order to prevent any flicker in an image plane owing to an after-image, there is required an aligning film having a high voltage holding ratio even at 60.degree. C. to 90.degree. C., at which temperatures a decrease in the voltage holding ratio is hitherto particularly eminent.
Furthermore, Vth (threshold voltage) around a liquid crystal panel tends to rise partly under an atmosphere of a high temperature and a high humidity (called Vth unevenness). Although the cause thereof is not certain, it is considerable that water and impurities in a sealing material are diffused and permeated in the liquid crystal aligning film to cause the phenomenon.
As aligning films used in the said liquid crystal display elements, organic films such as polyimide and polyamide types are mainly used, and a liquid crystal display element with a liquid crystal aligning film is disclosed in Japan Open-laid Patent Application Sho 51-65960, in which polyimide resin having a repeating unit represented by ##STR2## is used. However, in the said element using the polyimide aligning film, the image sticking and the Vth unevenness are easily produced.
Problems to be solved by the Invention
An object of the invention is to solve the above-mentioned problem as well as to propose diamino compounds which is suitable to obtain a raw material, a polyimide resin, for a liquid crystal aligning film with a less image sticking, with a high voltage holding ratio from a low temperature to a high temperature and without any Vth unevenness around the liquid crystal panel even under an atmosphere of a high temperature and a high humidity.
Means to solve the Problem
We inventors have gone on with studies and developments in earnest, and thus found that the image sticking, the voltage holding ratio, Vth unevenness are correlated to a polarity of an aligning film surface and that an aligning film with a less image sticking, with a high voltage holding ratio from at a lower temperature to a high temperature, and without any Vth unevenness around the liquid crystal panel even under an atmosphere of a high temperature and a high humidity can be obtained by using as the polyimide raw material for the aligning film a diamino compound with a specified structure which is able to decrease the polarity, to complete this invention.
Since the said diamino compound has not a polar group such as --O-- or --SO.sub.2 -- and also has a large molecular weight, the proportion of the imide group with a large polarity can be decreased relatively in the case that the compound is used as the polyimide raw material.